The invention relates to heat-cured compositions which are distinguished by both high impact strength and a high glass transition temperature at the same time, and which in particular can be used as one-component adhesives.
High-quality adhesives are increasingly used in manufacture of both vehicles and mounted parts or machinery and equipment, either instead of or in combination with conventional joining methods such as riveting and punching or welding. This results in advantages and new options for manufacturing, for example manufacture of composite and hybrid materials, or greater freedom in designing components. For use in vehicle manufacture, adhesives must have good adhesion to all substrates used, in particular electrogalvanized, hot-dip galvanized, and subsequently phosphatized sheet steel, lubricated sheet steel, as well as various types of aluminum. These good adhesion properties must also be maintained in particular after aging (alternating climate, salt spray bath, etc.) with no major loss in quality. If the adhesives are used as bodyshell adhesives, then the wash resistance of this adhesive is of considerable importance in order to assure process reliability at the manufacturer's facilities.
Wash resistance can be achieved with or without pregelling. To achieve sufficient wash resistance, the adhesive can be pasty and pregelled in a bodyshell oven within a short time or via induction heating of the parts to be joined.
Bodyshell adhesives must be cured under conventional baking conditions, ideally for 30 minutes at 180° C. Furthermore, they must also be stable up to about 220° C. Other requirements for such a cured adhesive or the bond are that operational reliability is ensured both at high temperatures up to about 85° C. and at low temperatures down to about −40° C. Since these adhesives are structural adhesives and thus these adhesives bond structural parts, high strength of the adhesive is of utmost importance.
Conventional epoxy adhesives are distinguished by high mechanical strength, in particular high tensile strength and high tensile shear strength. When the bond is 40° C. Since these adhesives are structural adhesives and thus these adhesives bond structural parts, high strength of the adhesive is of utmost importance.
Conventional epoxy adhesives are distinguished by high mechanical strength, in particular high tensile strength and high tensile shear strength. When the bond is subjected to impact loading, however, conventional epoxy adhesives are usually too brittle; and so under crash conditions, when both high tensile and high peel stresses occur, they are far from able to meet the requirements of the automobile industry in particular. They also have insufficient strengths at high temperatures and in particular at low temperatures.
Various approaches have been suggested to reduce the brittleness of epoxy adhesives under impact loading. Essentially two methods have been suggested in the literature for improving the impact strength of epoxy adhesives. First, the goal can be achieved by addition of at least partially crosslinked high molecular weight compounds such as latexes of core/shell polymers or other flexibilizing polymers and copolymers. Secondly, some increase in strength can also be achieved by introducing soft segments, e.g., by appropriate modification of the epoxy components.
According to the first method mentioned above, U.S. Pat. No. 5,290,857 and U.S. Pat. No. 5,686,509 describe how epoxy resins can be made impact resistant by mixing a fine powdered core/shell polymer into the epoxy matrix. This results in formation of highly elastic domains in the hard brittle epoxy matrix, which increase the impact strength. U.S. Pat. No. 5,290,857 describes such core/shell polymers based on acrylate or methacrylate polymers. U.S. Pat. No. 5,686,509 describes similar compositions based on ionically crosslinked polymer particles, where the core polymer consists of crosslinked diene monomers and the shell copolymer consists of crosslinked acrylic acid, methacrylic acid, and unsaturated carboxylic acid monomers.
According to the second method mentioned above, the U.S. Pat. No. 4,952,645 describes epoxy resin compositions which were flexibilized by reaction with aliphatic, cycloaliphatic, or aromatic carboxylic acids, in particular dimeric or trimeric fatty acids, as well as with aliphatic or cycloaliphatic diols. Such compositions should be distinguished by increased flexibility, in particular at low temperatures.
Modification of epoxy adhesives by means of polyurethane/epoxy adducts is also known. In this case, the terminal isocyanate groups of the prepolymers are reacted with at least one epoxy resin, where a hot-melt adhesive is obtained that is solid at room temperature. This method is described in EP 0 343 676.
It is also known that epoxy resins can be flexibilized with elastomers such as synthetic rubbers and their derivatives. The major effect in this case is based on the only partial miscibility of epoxy resins and the corresponding derivatized synthetic rubbers, where as a result heterodisperse phases are formed during the manufacturing process that have an effect comparable to the effect of core/shell polymers. Establishment of this superstructure depends on both the quantitative composition and on process control during the cure process. In the literature known to the person skilled in the art, carboxyl-terminated polybutadiene/acrylonitrile copolymers, which are reacted with epoxy resins, are described as particularly preferred starting compounds for this flexibilizing method. U.S. Pat. No. 5,278,257 and WO 0 037 554 describe epoxy adhesive formulations which contain as major components adducts with epoxy end groups, produced by reaction of carboxyl-terminated butadiene/acrylonitrile or butadiene/methacrylate compounds (or their styrene copolymers) with epoxy resins, as well as phenol-terminated polyurethanes or polyureas. Such adhesives can have high parameter values under peel, impact, or impact/peel loading.
A considerable drawback of the prior art is generally that by increasing the impact strength, the glass transition temperature and/or the strength of the adhesive is reduced; or that by raising the glass transition temperature, the strength is indeed increased as a rule but the impact strength, as well as the adhesion and especially the peel strength, is reduced. This situation severely limits use as a structural bodyshell adhesive, in particular because extremely high demands are made on a crash-resistant adhesive.
Additionally, the use of liquid rubbers is quite disadvantageous, since it means that the extent of phase separation and thus also the improvement of impact strength depends very much on the manufacturing or cure conditions, leading to considerable variations in the properties.